Plasma or dry etching is a well known technique used in the fabrication of integrated circuits wherein gas which has been dissociated into positive and negative ions and radicals reacts with unprotected areas of a material disposed on a substrate to remove selected portions of the material from the substrate. The etching generally takes place in a chamber containing the etching gas which is formed into plasma by the application of R.F. power in a controlled pressure environment. Also, etching or stripping may take place downstream from the plasma chamber by transporting the reactive species from the plasma chamber to an area containing the devices to be etched or stripped.
Probably the most common method of supplying power to an etching chamber is by direct coupling. In this method electrodes are disposed within the plasma chamber and directly connected to an A.C. power source. In such a method discharges occur between the electrodes which introduce process artifacts by sputtering, chemical reaction, release of entrapped and/or absorbed inpurities. These artifacts or contaminates are released in particle or gaseous form and settle onto the semiconductor device often destroying the usefulness of the device.
This problem has been overcome in the past by capacitive coupling in which the power source is indirectly coupled to the plasma chamber. By such coupling electrodes are not within the plasma chamber and the problem of electrode discharge and the contaminants formed thereby is overcome.
However, capacitive coupling between power source and plasma chamber introduces the necessity of using high frequency sources of power. This is so because as frequency is lowered in a capacitively coupled arrangement more of the power is dissipated in the coupling and less by the intended plasma load. Thus, in capacitive coupling the apparatus is efficient only at high frequencies. The necessity of using such high frequencies may cause damage or breakdown of the coupling structure since at high power requirements prohibitively large voltages appear across the coupling. In addition, the cost of such a system is high because high frequency power generators typically cost significantly more than low frequency power sources.
The present invention eliminates the disadvantages of capacitive coupling while also eliminating the problems associated with direct coupling.